Site-scale groundwater pollution risk assessment using surrogate models and statistical analysis.

Petroleum pollution in soil and groundwater has emerged as a significant environmental concern worldwide. As a sustainable and cost-effective in-situ remediation technique, Monitored Natural Attenuation (MNA) exhibits significant promise in addressing sites contaminated by petrochemicals. This study specifically targets a typical petrochemical-contaminated site in northern China and employs GMS software to establish a comprehensive physical model. The model relies on time-series monitoring data of phenol concentrations spanning from 2018 to 2020, effectively simulating both the leakage and natural attenuation of phenol. Within this study, the adsorption coefficient and maximum adsorption capacity emerge as the foremost influential factors shaping the outcomes of the model. Given the inherent heterogeneity of the site and the variability of hydrochemical conditions, parameters such as dispersion, porosity, and adsorption coefficient exhibit significant uncertainties. Consequently, relying on traditional deterministic models to predict the feasibility of MNA technology is not reliable. Therefore, this study employs machine learning (ML) methods to construct stochastic parameter models based on physical processes. The Random Forest Regression (RFR) algorithm, after trained, demonstrates strong alignment with numerical model output, exhibiting an average Nash-Sutcliffe Efficiency (NSE) >0.96. Using a stochastic approach, RFR iteratively computes phenol concentration across 6000 sets of parameters. Applying probability statistics, the model shows a notable reduction in the likelihood of phenol concentrations exceeding a threshold, dropping from 64.0% to 15.7% before and after natural attenuation. In parameter uncertainty, the stochastic model emphasizes natural attenuation's efficacy in mitigating phenol pollution risk (porosity being the most influential factor). This case study proposed a novel method to quickly assess the pollution risks at petrochemical sites under the influence of the uncertainty of pollutant transport and reaction parameters. The results can provide a reference for the pollution risk assessment at petrochemical sites, especially in sites with high stratigraphic heterogeneity or insufficient transport parameter data.

Determination of the binding affinities of OPEs to integrin αvß3 and elucidation of the underlying mechanisms via a competitive binding assay, pharmacophore modeling, molecular docking and QSAR modeling.

Organophosphate esters (OPEs) can cause adverse biological effects through binding to integrin αvß3. However, few studies have focused on the binding activity and mechanism of OPEs to integrin αvß3. Herein, a comprehensive investigation of the mechanisms by which OPEs bind to integrin αvß3 and determination of the binding affinity were conducted by in vitro and in silico approaches: competitive binding assay as well as pharmacophore, molecular docking and QSAR modeling. The results showed that all 18 OPEs exhibited binding activities to integrin αvß3; moreover, hydrogen bonds were identified as crucial intermolecular interactions. In addition, essential factors, including the -P = O structure of OPEs, key amino acid residues and suitable cavity volume of integrin αvß3, were identified to contribute to the formation of hydrogen bonds. Moreover, aryl-OPEs exhibited a lower binding activity with integrin αvß3 than halogenated- and alkyl-OPEs. Ultimately, the QSAR model constructed in this study was effectively used to predict the binding affinity of OPEs to integrin αvß3, and the results suggest that some OPEs might pose potential risks in aquatic environments. The results of this study comprehensively elucidated the binding mechanism of OPEs to integrin αvß3, and supported the environmental risk management of these emerging pollutants.

A national-scale assessment of land subsidence in China's major cities.

China's massive wave of urbanization may be threatened by land subsidence. Using a spaceborne synthetic aperture radar interferometry technique, we provided a systematic assessment of land subsidence in all of China's major cities from 2015 to 2022. Of the examined urban lands, 45% are subsiding faster than 3 millimeters per year, and 16% are subsiding faster than 10 millimeters per year, affecting 29 and 7% of the urban population, respectively. The subsidence appears to be associated with a range of factors such as groundwater withdrawal and the weight of buildings. By 2120, 22 to 26% of China's coastal lands will have a relative elevation lower than sea level, hosting 9 to 11% of the coastal population, because of the combined effect of city subsidence and sea-level rise. Our results underscore the necessity of enhancing protective measures to mitigate potential damages from subsidence.

Sources and transformation of nitrogen in shallow aquifers with progressive water table recovery using geochemical and isotopic approaches.

Groundwater level recovery has a significant effect on the sources and transformation of nitrogen in groundwater, but there are still few studies on the influences of the water table on the sources and transformation of nitrogen in groundwater using field data. In this study, the changes in groundwater level, geochemical composition, and isotopic signatures of NO3- and NH4+ during a period of groundwater level recovery at a pilot site were analyzed in detail. The water table underwent progressive recovery of almost 1.6 m in 16 months. At a depth of 5.5 m below the surface, both low NH4+ and high NO3- appeared in the groundwater, whereas below that depth, both high NH4+ and low NO3- simultaneously appeared in the groundwater. The main sources of NO3- were manure and septic waste, and NH4+ fertilizers. The main sources of NH4+ were mineral fertilizers. The main transformation process were nitrification and denitrification at a depth of 5.5 m below the surface; below that depth, the main transformation processes were denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This study provides a theoretical understanding of the relationship between the changing water table and nitrogen in groundwater level recovery areas.

Quantifying the contribution of ecological water replenishment on aquifer recovery using a refined groundwater model.

Due to its independent control and directly easy operation, ecological water replenishment (EWR) has been an important measure for restoring river ecosystems. However, the positive and negative contribution of the EWR activities to aquifer system are not fully understood under the combined influences of climate change and human activities across time scales. A refined groundwater flow model integrating an open channel flow at daily time scales is developed in a part of Northern China Plain to reproduce the dynamic process of groundwater level changes. After model calibration with groundwater level and runoff data, the changes of simulated groundwater level and river runoff have the Nash-Sutcliffe efficiency coefficient of 0.98 and 0.60, respectively. Results clearly demonstrate that the impulse response of aquifer recovery to runoff in three centralized EWRs. By using with and without EWR method, the simulated maximum contribution of EWR near river to aquifer recovery may be over 70 %. Scenario analysis method considering different precipitation, groundwater exploitation reduction and EWR activities are applied to evaluate the total quantities of aquifer recovery. The prediction of nine-year EWR activities under multiple scenarios shows that the increased groundwater level generally varies from 4.08 to 8.57 m, and the contribution of EWR accounts for 7.88 %-36.59 %. It is also noticed that 14 out of the 18 informal landfill sites will face potential groundwater pollution risks, indicating the negative influences of long-term EWR activities. This study can provide a method for quantifying the influences and contribution of EWR on aquifer recovery and can be referred to as a guideline for EWR evaluation with similar hydrogeological conditions.

Enhancing the understanding of hydrological responses induced by ecological water replenishment using improved machine learning models: A case study in Yongding River.

The ecological water replenishment (EWR) of Yongding River has been an important project implemented in response to the Development of an Ecological Civilization policy in China since 2016. A reasonable amount of EWR requires a systematic understanding of the relationship among the surface water, groundwater, ecology and economy. However, studying surface water-groundwater interactions still remains an important issue. Thus, a coupled model integrating a Muskingum method-based open channel flow model and machine learning-based groundwater model is developed to describe the dynamic changes in streamflow and groundwater level in response to the EWR of Yongding River. The model is calibrated using observed streamflow data as well as groundwater level data on a daily scale for the spring EWR in 2020. The simulated results match well with the observed data and suggest that significant groundwater level increases occur only around the main channel of Yongding River. Fifteen scenarios under different EWR schemes are set to obtain reasonable streamflow during EWR, and then the responses of streamflow and groundwater level changes are simulated. Reasonable streamflow at the Guanting Reservoir need to be above 65 m3/s to ensure the streamflow can pass through Beijing and significant groundwater level recoveries of 170 million m3 through EWR. The developed models can improve the understanding of the interaction between surface water and groundwater and provide a quick assessment of the factors influencing the different EWR schemes and thus aid in effective EWR project management.

Modeling the risk of U(VI) migration through an engineered barrier system at a proposed Chinese high-level radioactive waste repository.

The migration of U(VI) through the engineered barrier system (EBS) and into the natural environment in a geological repository for high-level radioactive waste depends on the chemical and physical environment of the repository. Modeling is widely used to understand the risk associated with migration of U(VI) for different barrier designs for repository sites. In this study, coupled thermal, hydrological, and chemical (THC) models were used to evaluate the risk of U(VI) migration at a proposed deep geological repository in northwestern China. The models incorporated two-site protolysis nonelectrostatic surface complexation, dissolution/precipitation of minerals and cation exchange as the major reactions controlling U(VI) migration. Modeling results showed that the main factors influencing U(VI) migration were pH, and the smectite content in the bentonite, as dissolution of the hydrous uranium oxide mineral schoepite is suppressed at higher pH values, and smectite is the most important adsorbent of dissolved U(VI). Therefore, an alkaline bentonite with a smectite volume fraction of >0.6 is suggested as the backfill material for this EBS. The THC model results also showed that in 100,000 years, U(VI) migration is constrained within EBS if the suggested bentonite is used as backfill in a repository that is hosted within Beishan granite. This study provides a feasible method for selecting a bentonite backfill and predicting the effect of environmental conditions on U(VI) migration.

Estimating dewatering in an underground mine by using a 3D finite element model.

Groundwater inflow to an underground mine will seriously affect its mining plan and engineering geology safety. Groundwater models are powerful tools commonly used in the mines to develop dewatering strategies. Many mines in the Kolwezi area have been present since the 1950s, and groundwater flow patterns have been significantly influenced by mining activities. A mining plan is developed for an underground mine with overturned syncline strata in Kolwezi, Congo. Previous groundwater models using layered homogeneous media lowered model accuracies. A new three-dimensional groundwater model using FEFLOW, consisting of a combined regionally and locally geology models integrating 16 hydrogeological cross-sections and borehole logging data, are formulated to predict the underground dewatering in the study area. A 31-days pumping tests with 3 pumping wells and 28 observation wells are carried out to estimate the hydrogeological properties. The simulated water level data match the observed data rather well. Under 8 scenarios of possible well designs, the model predicts a possible dewatering capacity greater 23,900 m3/d at the initial stage of mining. The concept of the model and its application can be a reference for other mines with complex geology for mining safety in the region of interest.

Groundwater nitrate pollution risk assessment of the groundwater source field based on the integrated numerical simulations in the unsaturated zone and saturated aquifer.

Groundwater pollution risk assessment in the groundwater source field (GSF) is crucial to ensure groundwater quality safety. A systematic method of assessing groundwater pollution in the GSF was established by combining the numerical models of groundwater flow and solute transport in the vadose zone and aquifer. It is featured by revealing the paramount fate of contaminant from the surface to receptor "well (wells)" via the pathway of vadose zone and aquifers. The method was verified in the phreatic and semi-confined aquifers of a vital GSF, Beijing-Tianjin-Hebei region (BTHR) in China. Nitrate was selected as the model pollutant. The results indicated that the groundwater pollution risk of the phreatic aquifer was dominated by the mediate level (45.27%), and that the second semi-confined aquifer was mainly ranked as relatively low (30.29%) and mediate (38.17%) levels. The groundwater pollution risk maps of the two aquifers were similar. The high and relatively high risk areas were affected by the high intensities of groundwater pollution sources (GPSIs) or short distances from the pollution sources to the pumping well. The low and relatively low risk areas were controlled by low GPSIs and adequate attenuation and denitrification of nitrate in the aquifer. The groundwater pollution risk in the semi-confined aquifer was lower than that in the phreatic aquifer. The groundwater pollution risk mapping provides a valuable scientific reference for the groundwater pollution prevention and control with the focus on the "pollution source" and "groundwater source field". The proposed method can be further applied to the protections of the GSFs in the BTHR.

Effect of colloids on non-Fickian transport of strontium in sediments elucidated by continuous-time random walk analysis.

Understanding the influence of colloids on radionuclide migration is of significance to evaluate environmental risks for radioactive waste disposals. In order to formulate an appropriate modelling framework that can quantify and interpret the anomalous transport of Strontium (Sr) in the absence and presence of colloids, the continuous time random walk (CTRW) approach is implemented in this work using available experimental information. The results show that the transport of Sr and its recovery are enhanced in the presence of colloids. The causes can be largely attributed to the trap-release processes, e.g. electrostatic interactions of Sr, colloids and natural sediments, and differences in pore structures, which gave rise to the varying interstitial velocities of dissolved and, if any, colloid-associated Sr. Good agreement between the CTRW simulations and the column-scale observations is demonstrated. Regardless of the presence of colloids, the CTRW modelling captures the characteristics of non-Fickian anomalous transport (0 < ß < 2) of Sr. In particular, a range of 0 < ß < 1, corresponding to the cases with greater recoveries, reveal strongly non-Fickian transport with distinctive earlier arrivals and tailing effects, likely due to the physicochemical heterogeneities, i.e. the repulsive interactions and/or the macro-pores originating from local heterogeneities. The results imply that colloids can increase the Sr transport as a barrier of Sr sorption onto sediments herein, apart from often being carriers of sored radionuclides in aqueous phase. From a modelling perspective, the findings show that the established CTRW model is valid for quantifying the non-Fickian and promoted transport of Sr with colloids.

Analytical methods for transient flow to a well in a confined-unconfined aquifer.

Concurrent existence of confined and unconfined zones of an aquifer can arise owing to ground water withdrawal by pumping. Using Girinskii's potential function, Chen (1974, 1983) developed an approximate analytical solution to analyze transient ground water flow to a pumping well in an aquifer that changes from an initially confined system to a system with both unconfined and confined regimes. This article presents the details of the Chen model and then compares it with the analytical model developed by Moench and Prickett (1972) for the same problem. Hypothetical pumping test examples in which the aquifer undergoes conversion from confined to water table conditions are solved by the two analytical models and also a numerical model based on MODFLOW. Comparison of the results suggests that the solutions of the Chen model give better results than the Moench and Prickett model except when the radial distance is very large or aquifer thickness is large compared with drawdown.